Right now, the only way I can see to do this is to delete the account and re-create it. This is a horrible usability problem.
Thanks for your report, but this is a duplicate of #219.
Please follow up here: https://forums.bitfire.at/topic/1144/owncloud-returns-invalid-pref-value
Thanks for the information. So this is now solved 🙂
OK. Your efforts are appreciated. Thanks. ☺